Membrane fusion mediates many profound biological events including fertilization, muscle development, synaptic transmission, and infections by enveloped viruses. The long-term objective of this research is to characterize the molecular mechanism(s) of viral and cellular membrane fusion proteins. The best characterized viral fusion protein is the influenza hemagglutinin (HA). Under fusion triggering conditions, the HA undergoes a conformational change which exposes a previously buried hydrophobic 'fusion sequence'. Insertion of this sequence into the target bilayer initiates fusion. The major goals of the proposed studies are (a) to determine to what extent this mechanism is used by other viral membrane fusion proteins and (b) to investigate if a crucial developmental fusion event--the formation of multinucleated contractile myotubes is mediated by a similar endogenous protein. The specific aims for the period covered by this proposal are: (i) to further characterize the molecular mechanism of membrane fusion induced by the influenza HA; (ii) to compare this mechanism with those of other viral membrane fusion proteins, notably the env glycoproteins of RNA tumor viruses; (iii) to identify a myoblast fusion protein. The overall approach is to apply biochemical and molecular biological techniques to study this cell biological process. The virus work will involve characterizing the interactions of viral fusion proteins with target bilayers. In addition, both natural and site-directed fusion mutants will be isolated. The genes encoding mutant fusion proteins will be cloned and expressed in eukaryotic cells for phenotypic and sequence comparison with their wild-type homologs. The muscle cell project will focus on the biochemical characterization of fusogenic vesicles isolated from rat myoblasts. The significance of the findings will be to deepen our understanding of the fundamental and widely occuring process of membrane fusion. Furthermore, since fusion is critically involved in infections by Influenza and mammary tumor viruses, as well as in normal muscle development, the work should have relevance for the control of viral diseases, tumors, and various muscle cell disorders.